The present invention relates to apparatus for and a method of obtaining a desired tint.
Such a tint may be used to alleviate visual discomfort in a number of circumstances. For example, flicker can cause seizures in up to 4% of patients with epilepsy. Such flicker may be caused by a television, because the lines that make up the pictures are scanned 25 times per second, and in addition the whole screen may flash 50 times per second owing to the frequency associated with the power source. Both sources of flicker can cause seizures in epileptics. Flicker is not noticeable at frequencies of about 100 flashes per second and beyond, but it nonetheless affects the firing of cells in the retina and parts of the brain. Such rapid flicker is present in a picture given by a visual display terminal, and also in the illumination produced by fluorescent lighting. Both of these sources affect eye movements. Fluorescent lighting in particular can cause eye-strain and headaches. With most fluorescent lamps the rapid flicker is greatest for the blue and green components of white light. So it has been found that rose-brown tinted spectacles reduce the flicker. Similarly such spectacles reduce the likelihood of headaches in people who use visual display terminals under fluorescent lighting, and also in children who suffer from periodic syndrome, which is a form of migraine.
Certain patterns can be unpleasant to look at. Such patterns may cause seizures in some patients with epilepsy who are sensitive to flicker. Such unpleasant patterns may cause anomalous perceptual effects, such as illusions of colour, shape and motion. However, some people are more susceptible to these effects than others. The discomfort and illusions from patterns are common in migraine, and may be related to immune system dysfunction.
Particularly bad patterns in this respect are striped, or have elements that form stripes. Successive lines of printed text resemble stripes (although such "stripes" are not as bad as continuous stripes). Some people are so sensitive to patterns that they see anomalous perceptual effects in text. Covering the lines of text above and below those being read reduces these effects.
Children with reading difficulty often have deficits on certain visual tasks and not others. The deficits suggest a dysfunction of the so-called "transient" system. Children with reading difficulties often report anomalous perceptual effects in text, but the relationship with the selective "transient" deficits is not yet know.
It has been found that some individuals no longer perceive the distortions described hereinabove when text is illuminated by coloured light, provided the colour is within a specific range that depends on the individual. The range of colours can be very small, and stable. It is unusual for the range to include shades of red. When colours with different saturation are compared, those with similar hue are preferred. There is usually also a range of colours that makes the distortions worse. Within this range discomfort or pain may occur. The discomfort can be assessed by observation of behaviour as well as subjective report. The uncomfortable colours are usually complementary to those that reduce perceptual distortion.
There has previously been proposed a technique for selecting appropriately tinted spectacles. Such children who seem to benefit from coloured overlays, are invited to undergo a one to two hour procedure to select an appropriate tint. Such a child goes through a large set of trial tints, picking out those that appear to make it easier for the child to view a page of text that has been written in a language which is foreign to the child. (A foreign text is used to emphasise the visual rather than the semantic aspects of reading). The examiner asks the child to describe the visual distortions he sees using his own words, and then employs this description when comparing the lenses. By a lengthy process of elimination the best tint is eventually selected and other tints added to fine-tune the colour. The selection eventually arrived at can involve a combination of three or more tinted trial lenses and the combination is often quite dark. This might be partly because one colour is being added to another to get the right tint. From this examination a code for the selected lenses is thus provided.
In order for patients to select a tinted lens it has been necessary in the past for the patient to view through varying numbers of coloured filters. A problem 10 with this method is the time-consuming number of operations involved in obtaining the desired tint.